In 1991, S. Iijima (Nature, vol. 354: 56-58) reported growth of multi-wall coaxial nanotubes, containing 2-50 layers with radial separations of about 0.34 nm, using an arc discharge evaporation method similar to that used for Fullerene synthesis. The nanotubes originally observed by Iijima were formed on the negative voltage end of a carbon electrode and were plentiful in some regions and sparse in other regions. Since that time, other workers have developed other discharge means for controlled deposition of graphitic carbon. However, it is not straightforward to control the growth of, or density of, single wall nanotubes (“SWCNTs”), multi-wall nanotubes (“MWCNTs”) and/or carbon-based nanofibers (“CNFs”).
Recently, interest has grown in use of arrays of carbon nanotubes (“CNTs”) as an intermediary for transport of electrical particles (e.g., electrons) and/or transport of thermal energy from one body to another. For example, a CNT array may be used for dissipation of thermal energy or accumulated electrical charge associated with operation of an electronics device or system. However, the device or system connected to the CNT array(s) may require use of different CNT array densities in different regions, because of differing transport requirements. Use of a mask to discriminate between a CNT growth region and a no-growth region has been demonstrated.
However, this approach only produces different regions where CNTs are present (with a substantially constant density) and where CNTs are absent (density substantially 0). Where maximum thermal transport is a focus, the desired CNT density is likely to be as high as possible, and no other limit is of concern. However, where electrical transport is a focus (e.g., between adjacent signal processing components on a semiconductor chip, the desired CNT density may lie in an intermediate range, with both a lower bound and an upper bound.
What is needed is an approach that allows control of CNT growth density on a coarse scale and on a fine scale simultaneously, preferably with two or more substantially different and adjustable scales (coarse and fine) for the CNT density. The CNT density is allowed to vary from one location to another, if desired. Preferably, the approach should allow variation and control, over a factor of about 1-1000, in the coarse scale local CNT density and should allow variation and control over a factor of about 1-10 in the fine scale local CNT density.